1. Field of the Invention
The present invention relates to a multilayered ceramic substrate and to a production method therefor. More particularly, the present invention relates to a multilayered ceramic substrate having a cavity for mounting and holding an electronic component therein, and to a production method therefor.
2. Description of the Related Art
In recent years, there has been an increasing demand for smaller and lighter electronic components having more functions, higher reliability, and the like. Consequently, there has been a need to increase the density of wiring on substrates.
In order to respond to the need for such an increase in density of wiring of the substrates, multilayered ceramic substrates produced by stacking a plurality of ceramic green sheets having conductive films and the like printed thereon, and by pressing and firing the ceramic green sheets, have been developed.
In order to reduce the size and thickness of the multilayered ceramic substrate itself, it is effective to form, in the multilayered ceramic substrate, a cavity for mounting an electronic component therein.
In such a multilayered ceramic substrate having a cavity, however, an end of a bottom face portion of the cavity is prone to cracking during the firing process. It is thought that such cracking occurs because it is difficult, due to the existence of the cavity, to press the entire green-sheet stack, which becomes a multilayered ceramic substrate, at a uniform pressure before the firing process, causing residual stress to be applied to the end of the bottom face portion of the cavity.
In order to solve the above problem, a method is disclosed in Japanese Unexamined Patent Application Publication No. 9-39160, in which a green-sheet stack having a cavity is vacuum-packed while being sandwiched between a pair of rubber sheets, and is subjected to isostatic pressing in a stationary fluid.
Japanese Unexamined Patent Application Publication No. 9-181449 discloses a method in which a green-sheet stack is pressed by an elastic member having a protuberance of the same shape as that of the cavity.
While both of the above methods aim to apply uniform pressure to the entire green-sheet stack, this is, in reality, quite difficult. For example, in the method disclosed in Japanese Unexamined Patent Application Publication No. 9-181449, it is necessary to minutely and strictly define the shape and properties of the elastic member to be used. Furthermore, it is almost impossible for any type of elastic member to apply a completely uniform pressure to the entire green-sheet stack.
That is, it is almost impossible to overcome the problem of the locally remaining stresses in the methods which attempt to apply a uniform pressure to the green-sheet stack having a cavity in the pressing process. For this reason, shrinkage stress is imposed in the firing process. Under the present circumstances, it is impossible, depending on the way in which the shrinkage stress is imposed, to completely prevent cracking from at the periphery of the bottom face portion of the cavity.
Accordingly, it is an object of the present invention to provide a method for producing a multilayered ceramic substrate which can overcome the above-described problems, and to provide a multilayered ceramic substrate obtained by the production method.
In order to achieve the above object, according to an aspect of the present invention, there is provided a method for producing multilayered ceramic substrate having a cavity including the steps of: forming a first ceramic green sheet having an opening for forming a cavity, and a second ceramic green sheet having no opening at least at a position corresponding to the opening; forming a green-sheet stack having a cavity defined by the opening by stacking the first ceramic green sheet and the second ceramic green sheet so that an aperture of the cavity is placed on at least one end face of the ceramic green sheets in the sheet-stacking direction; firing the green-sheet stack; and preparing a shrinkage-reducing material for reducing shrinkage stress produced at an interface between the first ceramic green sheet and the second ceramic green sheet, wherein the step of forming the green-sheet stack includes the step of making a shrinkage-reducing layer of the shrinkage-reducing material along the boundary interface between the first ceramic green sheet and the second ceramic green sheet so that the shrinkage-reducing layer is exposed at the bottom end of the inner peripheral surface of the cavity.
In this case, shrinkage stress produced at the end of the bottom face portion of the cavity during the firing process is reduced, and cracking will not occur thereat. This makes it possible to produce a highly reliable multilayered ceramic substrate.
Furthermore, since shrinkage stress produced at the end of the bottom face portion of the cavity during the firing process is reduced, as described above, it is unnecessary to minutely and strictly define the shape and properties of an elastic member if used in the step of pressing the green-sheet stack before firing. This makes it possible to reduce the equipment cost in the pressing step, and to enhance the efficiency of the pressing step.
Preferably, the shrinkage-reducing layer is exposed on the entire periphery of the inner peripheral surface of the cavity as a shrinkage-reducing pad.
Preferably, the area of the shrinkage-reducing pad is more than or equal to about 10% of the area of the principal surface of the first ceramic green sheet. Alternatively, the shrinkage-reducing pad is formed in a layer having a planar shape substantially identical to that of the first ceramic green sheet along the interface between the first ceramic green sheet and the second ceramic green sheet. This allows the shrinkage-reducing pad to reliably achieve the effect of reducing shrinkage stress.
Preferably, the thickness of the shrinkage-reducing pad is less than or equal to about 20% of the depth of the cavity. This makes it possible to prevent the side wall portion of the cavity from undergoing undesired deformation.
Preferably, the shrinkage-reducing pad contains a glass component, and the softening temperature of the glass component is less than or equal to the shrinkage starting temperature of the first and second ceramic green sheets.
Preferably, the first and second ceramic green sheets contain a glass component. In this case, it is preferable that the content of the glass component in the first and second ceramic green sheets is less than the content of the glass component in the shrinkage-reducing pad. This makes it possible to easily provide the shrinkage-reducing pad with required properties.
Preferably, the glass component contained in the shrinkage-reducing pad includes a constituent of the same type as that of the glass component contained in the first and second ceramic green sheets. In this case, it is more preferable that the glass component contained in the shrinkage-reducing pad be of the same type as the glass component contained in the first and second ceramic green sheets. In this case, it is possible to increase the bonding strength at the interface between the shrinkage-reducing pad and the first and second ceramic green sheets after firing.
The present invention is also applicable to a method for producing multilayered ceramic substrate using a so-called shrinkage-reducing process which substantially prevents a green-sheet stack from being shrunk in the direction of the principal surface thereof during a firing process.
Preferably, the multilayered ceramic substrate production method further includes a step of preparing a shrinkage-inhibiting inorganic material having a firing temperature greater than that of a ceramic material contained in the first and second ceramic green sheet. In the step of forming the green-sheet stack, a shrinkage-inhibiting layer containing the shrinkage-inhibiting inorganic material is formed to cover end faces of the green-sheet stack in the sheet-stacking direction while the opening is formed so as to expose the aperture of the cavity therefrom. The step of firing the green-sheet stack is carried on a condition that the ceramic material contained in the first and second green sheets is fired.
In this case, since first and second ceramic layers can be obtained from the first and second ceramic green sheets without substantially causing shrinkage in the direction of the principal surface, the multilayered ceramic substrate obtained is less prone to nonuniform deformation. For this reason, it is possible to prevent wiring conductors disposed in connection with the multilayered ceramic substrate from undergoing undesired deformation and strain, and to smoothly increase the density of the wiring conductors.
The multilayered ceramic substrate production method may further include a step of removing the shrinkage-inhibiting layer after the firing step.
The present invention is also applicable to a multilayered ceramic substrate obtained by the above-described production method.
According to another aspect of the present invention, there is provided a multilayered ceramic substrate having a stack to be obtained by a firing process, wherein the stack includes a first ceramic layer having an opening for defining a cavity, a second ceramic layer having no opening at least at a position corresponding to the opening, the first ceramic layer and the second ceramic layer being stacked, and a cavity defined by the opening and having an aperture on at least one end face in the sheet-stacking direction, wherein a shrinkage-reducing pad is formed along at least a part of an interface between the first ceramic layer and a second ceramic layer so as to be exposed at the bottom end of the inner peripheral surface of the cavity, and wherein the shrinkage-reducing pad contains a shrinkage-reducing material for reducing shrinkage stress produced at the interface between the first ceramic layer and the second ceramic layer during the firing process.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.